An electron is a negatively charged particle that makes up part of an atom. All basic elements are made up of electrons, protons, and neutrons. A fundamental concept in chemistry is the ability to determine how many electrons an atom contains. By using a periodic table of elements, this can easily be determined. Other important concepts involve how to find the number of neutrons and valence electrons (number of electrons in its outermost shell) in an element.

Steps

Method1

Determining Electron Number in Neutral Atoms

1

Obtain a periodic table of elements. This is a color-coded table that organizes all the known elements by atomic structure. Each element has a 1, 2, or 3-letter abbreviation and is listed along with its atomic weight and atomic number.[1]

Periodic tables can easily be found in chemistry books and online.

2

Find the element in question on the periodic table. The elements are ordered by atomic number and separated into three main groups: metals, non-metals, and metalloids (semi-metals). They are further grouped into families including alkali metals, halogens, and noble gases.[2] Every column of the table is called a group and every row is called a period.

If you know the details of your element, such as what group or period it is in, it will be easier to locate.

If you don’t know anything about the element in question, just search the table for its symbol until you find it.

3

Find the atomic number of an element. The atomic number appears in the upper left-hand corner, above the element symbol in the square. The atomic number defines the number of protons present in that particular element.[3] Protons are the particles in an element that provide a positive charge. Because electrons are negatively charged, when an element is in its neutral state, it will have the same number of protons as electrons.

For instance, boron (B) has an atomic number of 5, meaning that it has 5 protons and 5 electrons.

Method2

Determining Electron Number of Positively/Negatively Charged Ions

1

Find the atomic number of the element. The atomic number appears in the upper left-hand corner, above the element symbol in the square on the periodic table. The atomic number defines the number of protons present in that particular element.[4] Protons are the particles in an element that provide a positive charge. Because electrons are negatively charged, when an element is in its neutral state, it will have the same number of protons as electrons.

For instance, boron (B) has an atomic number of 5, meaning that it has 5 protons and 5 electrons.

2

Identify the charge of the ion. Adding and removing electrons from an atom does not change its identity, but it changes its charge. In these cases, you now have an ion, such as K+, Ca2+, or N3-. Usually, the charge is expressed in a superscript to the right of the atom abbreviation.

Because an electron has a negative charge, when you add extra electrons, the ion is more negative.

When you remove electrons, the ion becomes more positive.

For example, N3- has a -3 charge while Ca2+ has a +2 charge.

3

Subtract the charge from the atomic number if the ion is positive. If the charge is positive, the ion has lost electrons. To determine how many electrons are left, subtract the amount of charge from the atomic number. In this case, there are more protons than electrons.

For example, Ca2+ has a +2 charge therefore it has 2 fewer electrons than a neutral calcium atom. Calcium’s atomic number is 20, therefore this ion has 18 electrons.

4

Add the charge to the atomic number if the charge is negative. If the charge is negative, the ion has gained electrons. To determine how many total electrons there are, add the amount of charge to the atomic number. In this case, there are fewer protons than electrons.

For example, N3- has a -3 charge which means it has 3 more electrons than a neutral nitrogen atom. Nitrogen’s atomic number is 7, therefore this ion has 10 electrons.

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How do I calculate the number of electrons by looking at a periodic table?

wikiHow Contributor

It is the atomic number. However, if it has positive ion, then this electron number will go down (ie +2 charge means two electrons have been lost, so the electron/atomic number will go down by two) and vice versa.

Valence electrons are the electrons contained in the outermost shell. If you look at the periodic table and at the period numbers, that is the number of valence electrons. If the number is larger than 10, subtract 10 so you get two valence electrons. Example: Oxygen is in the 16th period. If we subtract 10 from 16, we get 6; therefore, oxygen has six valence electrons.

Oxygen is in the 16th period. If we subtract 10 from 16, we get six, which is the number of valence electrons. We do this for elements in periods 11 through 16. For periods 1 through 10, the number of valence electrons is the number of the period in which they are in.

To get from grams to electrons, you have to convert grams to moles by dividing by molar mass; then, you have to convert the moles into molecules by multiplying by Avagadro's number (6.02e23). As each water molecule has ten electrons, you multiply the number of molecules you have by 10.
In 250 grams of water, there are approximately 13.87 moles of water (found by dividing 250 by 18.02). Therefore, there are 8.35e24 molecules of water and finally 8.35e25 electrons (after multiplying the molecules by 10).